1. Field of the Invention
The invention relates to coordination amongst distributed computational systems and, in particular, to techniques for performing compound operations on memory in linearizable form.
2. Description of the Related Art
Computer systems often provide primitive instructions or operations that perform compound operations on memory in a linearizable form (i.e., as if atomically). For example, the VAX computer architecture provided instructions to directly support insert and delete operations on double-ended queues. Other processor architectures have provided simpler operations, such as a “test-and-set” operation (e.g., as provided by the IBM 360), a “fetch-and-add” (e.g., as provided by the NYU Ultracomputer), a “load locked” and “store conditional” operation pair (e.g., as provided by the DEC, now Compaq, Alpha), or “compare-and-swap” (e.g., as provided by the Motorola 68000 and processors conforming to the SPARC™ architecture).
SPARC architecture based processors are available from Sun Microsystems, Inc, Mountain View, Calif. SPARC trademarks are used under license and are trademarks or registered trademarks of SPARC International, Inc. in the United States and other countries. Products bearing SPARC trademarks are based upon an architecture developed by Sun Microsystems.
A “compare-and-swap” operation (CAS) typically accepts three values or quantities: a memory address A, a comparison value C, and a new value N. The operation fetches and examines the contents V of memory at address A. If those contents V are equal to C, then N is stored into the memory location at address A, replacing V. Whether or not V matches C, V is returned or saved in a register for later inspection (possibly replacing either C or N, depending on the implementation). All this is implemented in a linearizable, if not atomic, fashion. Such an operation may be notated as “CAS(A, C, N)”.
A more powerful and convenient operation is “double compare-and-swap” (DCAS), which accepts six values: memory addresses A1 and A2, comparison values C1 and C2, and new values N1 and N2. The operation fetches and examines the contents V1 of memory at address A1 and the contents V2 of memory at address A2. If V1 equals C1 and V2 equals C2, then N1 is stored into the memory location at address A1, replacing V1, and N2 is stored into the memory location at address A2, replacing V2. Whether or not V1 matches C1 and whether or not V2 matches C2, V1 and V2 are returned or saved in registers for later inspection. All this is implemented in a linearizable, if not atomic, fashion. Such an operation may be notated as “DCAS(A1, A2, C1, C2, N1, N2)”.
The SPARC version 9 architecture supports an implementation of CAS instruction on both 32-bit-wide and 64-bit-wide operands, but does not provide a DCAS instruction. On the other hand, the Motorola 68040 processor supports a double compare-and-swap instruction (on the 68040 it is called “CAS2”). Unfortunately, the CAS2 instruction effectively locks the entire memory system rather than locking first one location and then another. The net result is that deadlock is not possible but CAS2 instructions executed by separate processors are always serialized in time even if they access distinct memory locations.
There is substantial theoretical literature on the use of CAS operations and ordinary DCAS operations in the design of non-blocking algorithms. For example, Massalin and Pu proposed a collection of DCAS-based concurrent algorithms. They built a lock-free operating system kernel based on the DCAS operation offered by the Motorola 68040 processor, implementing structures such as stacks, FIFO-queues, and linked lists. See e.g., H. Massalin and C. Pu, A Lock-Free Multiprocessor OS Kernel, Technical Report TR CUCS-005-9, Columbia University, New York, N.Y. (1991).
Greenwald, a strong advocate of DCAS, built a collection of DCAS-based concurrent data structures, some of which improve on those of Massalin and Pu in terms of their properties and performance. In addition, he proposed various implementations of DCAS in software and hardware. See e.g., M. Greenwald, Non-Blocking Synchronization and System Design, Ph.D. thesis, Stanford University Technical Report STAN-CS-TR-99-1624, Palo Alto, Calif. (1999).
A drawback of the DCAS operation (as implemented by the Motorola 68040 processor and as described in the literature) is that it requires both of memory addresses, A1 and A2, to be valid. If either of the memory addresses A1 or A2 is invalid, then a memory fault occurs, typically resulting in an interrupt or trap. For certain kinds of algorithms that could otherwise profitably make use of the DCAS operation, such behavior is undesirable. For example, under certain conditions, the memory address A2 might be invalid or unusable, but only when the contents of memory address A1 are unequal to C1. 